1) Field of the Invention
The invention relates to lighting systems and, more particularly, to a device for reducing system wattage in gas discharge lamp lighting systems.
2) Description of the Prior Art
High pressure gas discharge (HID) lamps are widely used for industrial and shop lighting, among others. HID lamps are lamps which have a discharge vessel, for example of quartz or ceramic, which have a filling that supports a discharge arc at a gas pressure during operation generally at above about 2 atmospheres. High pressure sodium (HPS), high pressure mercury vapor and metal halide lamps are within the group known as HID lamps.
HID lamps, as with low pressure gas discharge lamps such as fluorescent lamps, have a negative resistance characteristic and require a stabilization ballast to control the current through the lamp during lamp operation. Without a ballast, the lamp current would increase rapidly and uncontrollably after lamp ignition, leading to failure of the lamp. The simplest ballast is a choke coil placed in series with the lamp and having an impedance chosen in accordance with the operating voltage of the lamp type for which it is designed to maintain the lamp current at the desired level. Such a ballast has an undesirably low and lagging power factor (current lagging the voltage). To improve the power factor, and also to reduce the starting current, a capacitor is placed in parallel with the choke coil. In the United States, ballasts used for HID lamps typically have a leading power factor (current leading the voltage) provided by a capacitor in series with the inductor. The above are the simplest ballast topographies. A very common ballast in commercial use for HID lamps is the constant-wattage autotransformer (CWA), which provides power stability despite common fluctuations in the mains voltage. This ballast includes a high reactance autotransformer (a transformer so connected that part of its winding is common to both the secondary and the primary) and a capacitor in series with the lamp, and provides a leading power factor.
Lighting accounts for approximately 20-25% of the electricity used in the United States. For stores, offices and warehouses, lighting may account for up to 50% of their electrical consumption. Accordingly, energy saving in lighting systems can provide a substantial savings in total energy usage for such commercial establishments.
Commercial HID lighting installations employ luminares dispersed throughout the area to be illuminated. A luminaire is a complete lighting unit which physically supports the ballast and its housing, the lamp socket and the lamp, and often a reflector to direct the light from the lamp. One way of achieving improved energy efficiency is to replace existing installations with new luminares having more efficient lamps and ballasts. For example, replacing luminares having a conventional mercury vapor lamp and CWA ballast with a luminaire having an HPS lamp and ballast designed for the HPS lamp will provide greater system efficacy. The disadvantage with this approach is the high initial capital cost.
Another approach is to replace only the lamp in the luminares with a more efficient lamp, which is a much lower cost alternative because the existing ballast and other luminaire components are retained. The lamp may be of a different type than that replaced. For example, it is common to replace mercury vapor lamps with HPS lamps, which have a higher efficacy than mercury vapor lamps of similar wattage and can operate on the same ballast. The new lamps may also be of the same type as that replaced, but modified to use less energy with the existing ballast. For example, one energy saving approach is to replace HPS lamps of one rated lamp voltage with HPS lamps of a lower rated lamp voltage. Generally, a decrease in rated lamp voltage of about 20% results in a lamp wattage decrease of about 10% when used with a CWA ballast.
While reducing lamp voltage results in energy savings, it has the disadvantage that the current through the lamp and ballast goes up. This causes higher ballast losses than with the original lamp and results in a considerably smaller decrease in system wattage than in lamp wattage. For a decreased lamp voltage of about 20%, the system wattage only decreases by about 5-7% for a decrease in lamp wattage of about 95%.
Accordingly, it is the object of the invention to decrease system wattage in retrofit gas discharge applications, i.e. without changing the existing ballast.